Power devices are widely used to carry large currents and support high voltages. Since the early 1950's, developers of electronic power systems began to base their power systems on semiconductor devices. Presently, many types of power semiconductor devices are available, including but not limited to power rectifiers, power bipolar transistors, power field effect transistors, power bipolar/field effect devices, power thyristors and other two, three or more terminal semiconductor devices.
Most power semiconductor devices being marketed today are fabricated in monocrystalline silicon. However, as is known to those skilled in the art, monocrystalline silicon carbide is particularly well suited for use in semiconductor devices, and in particular for power semiconductor devices. Silicon carbide has a wide bandgap, a high melting point, a low dielectric constant, a high breakdown field strength, a high thermal conductivity and a high saturation electron drift velocity compared to silicon. These characteristics would allow silicon carbide power devices to operate at higher temperatures, higher power levels, with lower specific on-resistance than conventional silicon based power devices. A theoretical analysis of the superiority of silicon carbide devices over silicon devices is found in a publication by Bhatnagar and coinventor Baliga entitled Comparison of 6H-SiC, 3C-SiC and Si for Power Devices, IEEE Transactions on Electron Devices, Vol. 40, pp. 645-655, 1993.
In order to take advantage of silicon carbide's higher breakdown field strength, it is important to create device edge terminations having breakdown voltages which approach the ideal parallel plane breakdown voltage. In particular, as is well known to those having skill in the art, for practical power devices, it is necessary to consider edge effects to obtain a realistic design. Edge termination limits the breakdown voltage of practical devices to below the theoretical limits set by semi-infinite device analysis. If a device is poorly terminated, its breakdown voltage can be as low as 10-20% of the theoretical case. This severe degradation in breakdown voltage can seriously compromise device design and lead to reduced current ratings as well. Accordingly, effort has been focused on the proper termination of the device region of power semiconductor devices.
Termination schemes for devices fabricated in a particular semiconductor material often cannot be used to terminate devices formed in another semiconductor material. For example, in silicon, many power device edge terminations have been proposed. These power device edge terminations are described, in detail in a textbook entitled Modern Power Devices by coinventor Baliga, published by John Wiley & Sons, 1987, at pp. 79-129. As described, terminations based upon planar diffusion are most commonly used for lower and medium power devices due to their process convenience. Unfortunately, these terminations are difficult to fabricate in silicon carbide due to the difficulty of forming P-N junctions in silicon carbide by conventional ion implantation and diffusion methods. Another well known silicon device termination technique physically changes the device edge by forming a beveled edge or a mesa edge. Again, however, it may be difficult to form these geometric features in silicon carbide due to the difficulty in etching silicon carbide and of removing the damage caused by the etching process.
A technique for improving breakdown voltage of gallium arsenide junctions is described in a publication by Shimamoto et al. entitled Improvement of Breakdown Voltage Characteristics of GaAs Junction by Damage-Creation of Ion-Implantation, published in the Institute of Physics Conference Series, No. 120, Chapter 4, pp. 199-202, 1992. There, ion implantation is performed around a junction edge to create implantation damage. An appropriate annealing step is then performed.
Several techniques for terminating silicon carbide devices have also been proposed. For example, floating metal field plate termination was found to improve the breakdown voltage from 210 volts to 400 volts and resistive field plate termination was found to improve the breakdown voltage to 500 volts in an article by Bhatnagar et al. entitled Edge Terminations for High Voltage SiC Schottky Barrier Rectifiers, published in the International Symposium on Power Semiconductor Devices, 1993 Proceedings, Abstract 4.2, pp. 89-94, 1993. Since the ideal parallel plate breakdown voltage for silicon carbide is about 900 volts, these edge termination techniques produce a breakdown voltage of about 60% of the ideal value. Floating field ring and floating field plate terminations for silicon carbide power MOSFETs are also described in U.S. Pat. No. 5,233,215 to coinventor Baliga.
In view of the above, there is a continuing need for device terminations and fabrication methods which are uniquely suitable for silicon carbide and which allow device breakdown voltage to approach the ideal value.